FIG. 4 is a planar view of a cylinder head of a conventional precombustion-chamber gas engine as seen from below. This conventional precombustion-chamber gas engine 100 includes a precombustion chamber cap 108 disposed inside the cylinder head 104 at the center of the cylinder head 104, as illustrated in FIG. 4. Two intake ports 113 and two exhaust ports 115 are opened around the precombustion chamber cap 108 of the cylinder head 104 so as to surround the precombustion chamber cap 108. In FIG. 4, two intake ports 113 are opened on the right side of the drawing, while two exhaust ports 115 are opened on the left side of the drawing.
Furthermore, the above two intake ports 113 and two exhaust ports 115 are opened and closed by intake valves 112 and exhaust valve 114, respectively. The intake valves 112 open and close the intake ports 113 so that mixed air is supplied to the main combustion chamber 110 delimited between the cylinder head 104 and a top face of a non-illustrated piston. The exhaust valves 114 open and close the exhaust ports 115 so that exhaust gas in the main combustion chamber 110 is discharged.
A precombustion chamber 120 is delimited inside the precombustion chamber cap 108. The precombustion chamber cap 108 includes a plurality of nozzle holes 116 that brings the precombustion chamber 120 into communication with the main combustion chamber 110. Six nozzle holes 116 are provided at equal intervals in the circumferential direction. The combustion flames “f” (hereinafter, referred to as torch jets) produced in the precombustion chamber 120 are injected into the main combustion chamber 110 through the nozzle holes 116, thereby combusting mixed air in the main combustion chamber 110. The arrows here of the torch jets “f” in FIG. 4 indicate the injecting direction and the strength of the penetration force of the torch jets “f”. The torch jets “f” in the drawing indicate that they are injected evenly in terms of space and at the same penetration force inside the main combustion chamber 110.
Patent Document 1, referred to later, is Japanese Unexamined Patent Application No. 2004-251213.